Abstract: Motor housing decorative cover is a critical housing component on motors. It is a circular aluminum alloy part with a large height difference, making feeding difficult and prone to problems such as insufficient filling and flyaways. Gating system utilizes a "Y"-shaped comb structure, with a built-up ingate to address filling issues. A ring-shaped overflow system and addition of a sealing plate address flyaways. To ensure product appearance quality, cavity and core are designed as a single piece. To address problem of product sticking to cavity, an undercut ejector pin is added to ensure smooth demolding.

Motor is a vital device for converting electrical energy into mechanical energy, playing an irreplaceable role in industrial development and daily life. To meet increasingly severe energy and environmental challenges, motor production requirements are becoming increasingly stringent, with high efficiency, intelligence, and environmental protection being key development directions. Currently, majority of motor housings, decorative covers, and other parts are produced using aluminum alloy die-casting. Die casting technology has advantages of high production efficiency, high casting precision and good surface quality. It has become an important process in modern manufacturing, especially in mass production. Taking motor housing decorative cover as an example, die casting process design and mold design are carried out to provide a reference for production of similar die castings.

Figure 1 is a part drawing of motor housing decorative cover. Material is ADC12 aluminum alloy, which complies with JIS-H5302-2006 standard. It has good fluidity and good mechanical properties. Casting size is ø266.5 mm x 70.4 mm, wall thickness is 3.5 mm, projected area is 557.8 cm, and single piece weight is 814.4 g. Radial dimension tolerance of casting is 0.3 mm, surface roughness R≤3.2 μm, 8 small holes evenly distributed on casting and through hole in the middle are obtained through subsequent processing. Cracks, bubbles and any penetration defects, as well as wear, depressions, missing meat and mesh burrs are not allowed in casting. One-third of casting's perimeter is hollow, and height difference is 66.4mm. This hinders molten metal filling, making underfill and flyaway material highly likely. This complicates die-casting process and mold design. Therefore, a rational design of gating and overflow systems, as well as adjustment of appropriate die-casting process parameters, is required.

 

Figure 1 Motor housing decorative cover parts diagram

Motor housing decorative cover has a relatively simple appearance. Parting surface is located at maximum contour dimension and varies along casting wheel, as shown in Figure 2. Because one-third of casting's perimeter is hollow, part is prone to insufficient clamping force and sticking to front mold during demolding. During mold design, attention should be paid to increasing tension on part from movable mold side. Fixed mold consists of a cavity insert and a fixed platen, while movable mold consists of a movable insert and a movable platen. To ensure precision during mold closing, positioning mechanisms are added to four corners of insert. Molded parts are shown in Figure 3.

 

Figure 2 Design of parting surface of motor housing decorative cover

 

Figure 3 Molded part

Design scheme for gating and overflow systems is shown in Figure 4. Gating system consists of a sprue, runner, and gate. Gate is located in hollow space at the top of decorative housing.Runner adopts a "Y"-shaped comb structure, and gate uses a built-up ingode. Thirteen overflow troughs are arranged in a circular pattern around edge of decorative housing. Typically, gate is located below shell. If gate is located at the bottom edge, liquid will eventually fill hollow space of decorative housing, which can easily cause flash. Therefore, inlet gate is located at the top to ensure product's appearance quality. Main purpose of "Y"-shaped comb structure is to achieve zoned filling through multiple branching runners and ingodes, ensuring that molten alloy is delivered to each ingode as evenly as possible and maximizing coverage of molten alloy entering mold cavity. This design helps improve filling quality and reduces defects that may occur due to unstable molten metal flow. Because "Y"-shaped comb-shaped runner structure evenly distributes molten metal to each ingate, it helps achieve uniform filling of all parts of mold cavity. This not only reduces risk of defects such as air holes and shrinkage cavities within casting, but also improves dimensional accuracy and surface quality of casting.

 

Figure 4. Casting system and overflow system of motor housing decorative cover

Cross-sectional area Ag of ingate is calculated as follows

 

Where: m is mass of molten metal passing through ingate, p is density of liquid alloy, v is flow rate of molten metal at ingate, and t is filling time.
Mass of motor housing decorative cover is 744 g, and density of ACD12 aluminum alloy is 2.7 g/mm2. Based on experience with similar thin-walled parts, v is taken as 50 m/s and t is taken as 0.1 s. Substituting into formula (1), Ag = 55.11 mm2 is obtained. Number of ingates is 4, which are fan-shaped, have dimensions of 41.1 mm x 1.55 mm and 32.2 mm x 1.55 mm, respectively. Ingate dimensions are shown in Figure 5. Main runner and branch runner are set on fixed mold side, and ingate is set on movable mold side. They are connected in a side plane. Transition angle between branch runner and gate is 16°. Cross-runner's deflection allows molten metal to enter cavity from side, which can effectively reduce turbulence and air entrainment during injection process.

 

Figure 5 Ingate size

Since gate of product is designed at upper edge of product, high metal alloy pouring temperature causes mold to heat up rapidly during die casting process. Thermal expansion of mold material close to cavity surface is greater than that of mold material far from cavity surface. Expansion of mold material near parting surface is less constrained than that deep in cavity. Molten metal is easy to splash out from parting surface under high pressure, so a sealing plate is designed in front of fixed mold and movable mold. Design of sealing plate needs to be optimized according to specific mold structure and production conditions to ensure that it can effectively prevent splashing of molten metal without affecting quality of casting. According to characteristics of large height difference of product, two upper and lower sealing plates are designed, and diverter is designed in upper sealing plate. Structure of sealing plate is shown in Figure 6.

 

Figure 6. Material distribution plate structure
1-Split cone 2-Locate ring 3-Gate bushing 4-Upper sealing plate 5-Fixed plate 6-Core 7-Moving plate 8-Lower sealing plate

Overflow system is mainly composed of an overflow trough and an exhaust trough. Overflow system has a great impact on quality of casting product. Exhaust will affect flow speed and flow direction of molten metal. Design of overflow trough directly affects appearance and strength of casting. Main function of overflow trough is to flush out oxides, lubricant residues and cooler parts of liquid from mold cavity. Overflow trough is usually located at the place where metal liquid finally arrives and at confluence of multiple streams of metal liquid. Since casting is filled with four gates, 13 overflow troughs are arranged in a circular pattern around edge of decorative cover according to area of cavity last filled with gate. Overflow cross-sectional area of all overflow troughs should be 60% to 70% of cross-sectional area of inner gate. Mold uses clearance between core and dynamic plate for exhaust.

Motor housing decorative cover is a thin-walled product with high requirements for product appearance. During ejection process, product is not allowed to be deformed due to uneven ejection force. Therefore, 15 ejectors are evenly arranged around circumference of product. Ejector pin distribution is shown in Figure 7. At the same time, ejectors are also arranged at diversion channel and overflow trough. During mold trial, die-casting part adhered to fixed die. This was due to simple surface structure of decorative cover, which resulted in clamping force on movable die being less than friction force required for demolding from fixed die. To increase clamping force on movable die, an undercut was added to top of ejector pin, allowing for smooth demolding. Undercut design is shown in Figure 8.

 

Figure 7 Ejector pin distribution diagram

 

Figure 8 Undercut design

Motor housing decorative cover was mass-produced on a 500t die-casting machine. Actual die-casting process parameters were: slow shot speed 0.2 m/s, fast shot speed 3 m/s, initial mold temperature 290℃, initial ADC12 aluminum alloy melt temperature 650℃, shot time 2 seconds, and cooling time 3.5 seconds. After mass production verification, product yield reached 98%, with no defects such as cracks, bubbles, or flash.

"Y"-shaped comb gating system, designed based on structural characteristics of casting, facilitates filling and forming, reducing undesirable appearance defects such as bubbles and flash. Integral cavity forming structure helps ensure appearance accuracy of product, while increasing four corner positioning to ensure mold closing accuracy. Undercut design at ejector ensures smooth demoulding of product.